Hair dye compositions for gradually coloring or darkening gray hair, usually referred to as progressive hair dyes, are well known in the art. Some of these compositions have been based on lead acetate. In order to avoid the use of lead in these products, bismuth salt based compositions have been described which have been considered as alternatives. The bismuth formulations have. been considered to be inferior to the lead acetate formulations based on the quality of color they produce. These compositions included methyl pyrrolidone as a catalyst for fixing the bismuth salt to the hair shaft as a bismuth sulfide complex which is formed in situ by reaction of the bismuth salt with a thiosulfate. Compositions which contain methyl pyrrolidone are described in U.S. Pat. No. 4,583,986 which is incorporated by reference and other compositions of bismuth salt based hair dyes are disclosed in U.S. Pat. No. 4,310,329 which is also incorporated by reference.
Methyl pyrrolidone is under regulatory review in Europe and for this reason, this product is not desirable for long-term use as a component of a hair dye for human use. Accordingly, there is a need for an alternative catalyst for transforming a bismuth salt to a bismuth sulfide complex in the presence of a thiosulfate.